© Galloway Astronomy Centre 2016
All images are copyright – M Alexander unless otherwise stated
Improving Your “Cheapy” Telescope
Low cost telescopes of say £50 to £120 don’t
tend to be very good quality. These are the
type you can buy in department stores, some
camera shops, ebay and even supermarkets.
If you have one of these there are a few things
you can do straight away to improve it.
All the parts as indicated in Image #1 that
make up a telescope are important not just
the tube you look through.
A brief extra note - in the Image #1 you will
see a ring with a hole in it at the front of the
telescope. If after you remove the front cover
you still see this on yours stick your finger in
the hole and hook out the rest of the cover. It
should pull away easily. The front of your
reflector should look like image #2. The whole
of the cover should be removed every time you
use the telescope. (I only mention this as I see
it all the time)
Cheap telescopes often have Altitude - Azimuth
mounts (AZ mount) which can be a bit flimsy
and wobbly. See Image #1. These points will
improve an equatorial mount (the type with a
counter weight) too.
The wobble is mostly due to the tripod legs not
being held rigidly. Avoid extending the tripod
legs too high as this makes the wobble worst.
The thin metal leg braces and accessories tray
which should help stabilise the mount often have
little effect. Also avoid using the telescope on a hard surface like
concrete, on grass push the spikes on the end of the legs into the
Try each of the following one at a time as just one might be
- Replace the tray with a larger triangular piece of 2 mm thick
plywood (shaped as in Image #3) this will hold the
legs rigid. The flat corners need to be the width of each tripod
- Tighten all wingnuts and screws on the tripod firmly without
over tightening to minimize flex in the whole structure.
- To minimize vibrations hang a large plastic container with a
handle like the ones for milk or fabric conditioner and half fill it
with sand. Now hang it from the centre of the mount.
By the way, you should not really use the tray to hold your
eyepieces - it allows them to get cold and damp.
Better to keep them in your pocket with the covers on
Your telescope is almost certainly has a tiny
finderscope. As you can see from the picture
(image #4) it looks like a mini telescope
mounted on top of the telescope itself. There is
are hidden problems with this type of
1) The problem is the small lens visible at the
front of the finderscope. Being small it does not let in very much
light which you need to be able to locate the objects you want to
2) Worst still if you look closely just behind the lens with be a disc
which is further restricting light entering your eye. It is there to
stop bright objects like the Moon and planets from having a
rainbow of colours around them as the lend is of poor quality.
However, the colours are not really much of a problem, better to
let all the light in.
Removing the baffle is very easy to do
following these steps:
- First unscrew the end you view through from
- Looking inside the now open end of the tube
you will either see a small black disc close to
the lens or a short black tube (see image #5)
- Now unscrew the front fitting that hold the
lens in place. Do this with the tube vertical and
the lens end at the bottom. This is so the lens does not fall out.
Don’t worry if it does, looking at it you will see the lens is flat on
one side and curved on the other. Drop the lens back in with the
curved side down.
- Now using a pencil or metal rod that is long enough you can
knock out the disc (or tube) baffle.
- Reattach the front lens end then the viewing end.
- Looking through the finderscope it should be noticeable how
much bright is the view.
You may not be aware that the finderscope can
be focused. From Image #6 you will see that the
end you look through rotates in both directions
separately to the part that you detached to get
inside to find the baffle. To focus the finderscope
point it toward a distant object and rotate the
lens end until the image is sharp.
The focuser is the part into which you put the eyepieces to view
through the telescope (see Image #1). There is a small knob on
either side which rotates to move the eyepiece in and out. This
can feel stiff which will cause you to shake the telescope making
accurate focusing difficult. Between the two knob on the
underside will be two (or four) cross headed screws. Unscrew all
the screws by about a half turn, that should give a smoother feel
to the focus. If you unscrew them too far you will see the focuser
knobs are able to wobble up and down. Just tighten the screws a
little until the wobble stops.
Eyepieces (please don’t call them lenses)
are extremely important as they provide the
magnification you need to view the object.
There are several common problems with
the eyepieces supplied with cheap
1) Some eyepieces are non standard 0.965”
diameter. They should be 1.25” diameter.
See Image #7
2) they have often tiny lenses so you have
to get your eye very close to the lens look
through it. This is a particular problem if you
3) the magnifications they achieve do not always give the best
If the instruction sheet does not tell you the diameter of the
eyepieces you can easily check by holding a ruler across the
chrome tube of the eyepiece. If it is less than 1.25” then you
have 0.965” eyepieces. These need to be replaced as soon as you
can. See the bottom of this page for alternative eyepieces.
Magnification (M) can be calculated by
M= focal length of the telescope
focal length of the eyepiece
For example a telescope of 700mm focal length (FL) and an
eyepiece of 4mm gives you an M = 175 (ie 700/4). This is
written as 175x - meaning 175 times magnification
For a telescope of 70 or 76mm diameter this is much too high.
The image will be faint and probably will not focus sharply.
The good guide to the maximum magnification of any telescope
can be found by dividing the aperture (telescope front lens or
mirror diameter) by 25 then multiply by 30
eg 76mm reflector telescope
76/25 = 3
3 x 30 = 90
If your maximum magnification does not exceed 90 your
telescope will give you enjoyable bright, sharp views.
Use the first calculation to see what magnification each of your
eyepieces will give. Your kit may also include a 2x (or 3x) Barlow
lens. This is a special lens which when used with an eyepiece
doubles (2x) or triples (3x) the normal magnification of that
Here is an example of the above:
You have a 76/700 reflecting telescope - this means the mirror is
76mm in diameter with a focal length of 700mm
There are 3 eyepieces with labels reading 20mm, 12.5mm &
4mm - this means focal lengths of these eyepieces are 20mm
12.5mm & 4mm
There is also a 2 x Barlow - this will triple the magnification of
any of the eyepieces
20mm + 2x Barlow
12.5mm + 2x Barlow
4mm + 2x Barlow
You may think a magnification of 35x will not show you much, but
it depends on the size of the object. The Moon will show a lot of
detail and allow you to decide where you what to look more
closely. If you have not too much light pollution large star
clusters will fill the view.
As you will see the 4mm eyepiece on its own far exceeds the 90x
maximum for this size of telescope. Although the use of the
Barlow lens with the 12.5mm eyepiece exceeds this, on bright
objects like the Moon you may find the view is OK.
A quick calculation will show that a 20mm eyepiece with a 3x
Barlow lens gives 105x magnification, which means you could not
use it with any of the other eyepieces.
A quick search of the internet for Eyepieces will soon show you a
confusing number of types and focal lengths, so which one is best
for you? First off don’t spend a huge amount on an eyepiece. An
eyepiece costing as much as your telescope will not improve the
view as much as you might hope. It is more sensible to save that
money for a better telescope in the future. Nor do you need a box
full of eyepieces - two or at most three covering low, medium and
high magnification is all you need. If you are happy with the
20mm eyepiece that came with the telescope why replace it? It is
the mid and high magnification eyepieces and probably the
Barlow lens too that usually need replacing.
So what other sizes do you need? You can work it out by
reversing the magnification equation.
Eyepiece FL = Focal length of the telescope
Eg You want 90x magnification
700/90 = 7.7mm
Eyepieces FL are usually whole numbers so you cannot buy a 7.7
mm eyepiece only a 7mm or 8mm. Rather than buy one that size
you can achieve the same magnification using the Barlow lens
and you get the advantage of the lower magnification using it on
its own too. Therefore, I would suggest a 15mm eyepiece. This
gives 47x on its own and 94x with the Barlow lens.
If the 20mm eyepiece is just adequate, but you would like to
change it then buy a 25mm eyepiece as it will give you lower
magnification and a larger field of view to locate an object. A
25mm eyepiece gives 28x on its own and 56x with the Barlow
lens. This later is the same as the 12.5mm eyepiece that came
with the telescope so you have effectively replaced that too.
So with just two new eyepieces and possibly a new Barlow lens
you have a wide range of magnification from low to the maximum
the telescope can easily handle.
The industry standard for eyepieces is a barrel diameter of 1.25”
so they can be used in any telescope you buy in the future.
How much do eyepieces cost?
We have a special offer :
Basic 2x Barlow Lens
Higher Quality Barlow Lens
Delivery charges apply
If you need any help or advice on modifications, eyepieces or
buying a new telescope please contact me.
Tel: 01988 500594